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TPS71533EVM: TPS71533DCKR - How to calculate Hold up time for the linear regulator

senthil kumar62
Prodigy 50 points
Part Number: TPS71533EVM

Hi

I'm using the TPS71533DCKR , I need to calculate the hold up time for the linear regulator, I'm using 1 uF capacitor as the load capacitance, what will be the hold up time. kindly provide me the formula for calculating the Hold up time for Linear regulator and switching regulator

Thanks,

P.Senthil Kumar 

  • 0
    Guru 140200 points
    Hi Senthil,

    what do you mean by "hold up time"?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points
    Hold Up time -Hold-Up time is defined as the duration of time that a power supply's output will remain within regulated limits following a loss of input power
  • 0 in reply to senthil kumar62
    Guru 140200 points
    Hi Senthil,

    you can estimate the charge delivered by a cap. If the current "I" is flowing for a time interval "dt", a cap of capacitance "C" will show a voltage drop of "U" according to:

    C = dQ / dU = I x dt / dU

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points
    HI
    Thanks for your reply
    I need to calculate for below condition
    Vin= 12V
    Vdrop=8V
    Vout=3.3V
    Iout=55mA
    Cout= 1uF

    what will be the hold time, i.e Absent of input voltage, how mush time the steady output voltage will present

    Thanks in advance
    P.Senthil kumar
  • 0 in reply to senthil kumar62
    Guru 140200 points
    Hi Senthil,

    this would need a huge output cap. Even with a 100µF output cap the output voltage will drop from 3.3V to 3.2V in only about 200µs, assuming a load current of 55mA.

    Using a high input capacitance will better do the trick. With a 100µF input cap the input voltage will drop from 12V to 4V in about 14.5ms, assuming a load current of 55mA.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points
    hi
    if you don't mind can u explain me in breify
    Thanks
    Senthil kumar
  • 0 in reply to senthil kumar62
    TI__Guru 55940 points
    Hi Senthil,

    What is pointing out is that the "hold time" is based on the discharge of your output capacitor. The primary discharge path of the output capacitor will be your load itself. You can use the equation to calculate the capacitance required for your discharge time at your given load current.

    As Kai points out, an alternative to a large output capacitor is place a large input capacitor so that the LDO is powered longer after the input supply is removed. As you have ample headroom (Vin - Vout), you could place a diode before the input capacitor so that the discharge of the input capacitor is through the LDO powering the load. This will actually help protect the LDO from the reverse current you would otherwise have due to only a large output capacitor as reverse current will flow when Vout exceeds Vin.

    Very Respectfully,
    Ryan